Water take-off and landing of amphibious aircraft depends on the planing performance of support floats (for a float plane) or the main fuselage (for a flying boat).
By definition, planing situates the aircraft atop the sea-way. In this position, the aircraft is subject to the motions of either following atop or “skipping” over the undulating wave surface. This often results in an unstable surface dynamic response, such as “porpoising” motion and even premature ballistic launch off of wave crests. These behaviors can result in near-surface stalls and crashes.
As a consequence, active pilot control and a calm sea-surface environment is required for water take-off and landing. And even with pilot control, take-off and landing is limited to sea states of no more than a few feet (i.e., SS1 or SS2) with conventional present day amphibious aircraft. Autonomous take-off and landing, such as would be required for an unmanned amphibious vehicle, is, for all practical purposes, impossible with present technology.